Sulfur recovery process using metal oxide absorbent

ABSTRACT

Sulfur species are removed from a Claus plant tailgas stream by contacting with ZnO producing ZnS. ZnS is regenerated to ZnO by contacting with dilute O2 under conditions for reducing the volume of regeneration recycle to the Claus plant.

This is a Continuation of copending application Ser. No. 195,426, filedon May 11, 1988, and now abandoned, which is a Continuation of Ser. No.893,111, filed Aug. 4, 1986, and now abandoned.

FIELD OF THE INVENTION

The invention relates to the removal of sulfur and sulfur compounds fromgaseous streams containing such compounds. In one aspect, the inventionrelates to the removal of sulfur compounds including H₂ S (hydrogensulfide) and SO₂ (sulfur dioxide) from Claus plant tailgas. In anotheraspect, the invention relates to the use of solid high surface areacontact materials (absorbents), for example, ZnO-based (zincoxide-based) absorbents, for absorbing sulfur compounds such as SO₂ andH₂ S. In a further aspect, the invention relates to the removal ofsulfur compounds from Claus plant tailgas where the Claus plant has atleast one Claus catalytic reaction zone operated under conditions,including temperature, effective for depositing a preponderance ofelemental sulfur on catalyst therein, and also where regeneration ofsulfided absorbent occurs at a diluent rate effective for reducing thevolume of regeneration effluent returned to the Claus plant.

SETTING OF THE INVENTION

A developing area of sulfur recovery technology is that of tailgascleanup, that is, of removing trace quantities of sulfur compounds fromgaseous effluent streams (tailgas) of Claus process sulfur recoveryplants. Tailgas may contain substantial amounts of sulfur compounds.Tailgas from Claus or extended Claus plants (having at least one Clauslow temperature adsorption reactor )typically can contain about 0.5-10%of the sulfur present in feed to the plant as elemental sulfur, H₂ S,SO₂, COS (carbonyl sulfide), CS₂ (carbon disulfide), and the like.Tailgas cleanup processes remove at least part of such residual sulfurcompounds from Claus tailgas.

In prior U.S. Pat. No. 4,533,529, Claus tailgas is contacted with ZnO(zinc oxide) in an absorber reducing average overall emission levelsfrom the absorber to less than 250 ppm sulfur species. It is desirable,however, and necessitated by certain environmental requirements, thatnot only average but instantaneous emissions be continuously maintainedat a very low level.

It has been discovered, after ZnS (zinc sulfide) is regenerated to ZnO,that an increase in SO₂ emissions occurs from the absorber uponreturning regenerated ZnO to absorption. These SO₂ emissions interferewith continuously maintaining instantaneous emissions at a very lowlevel.

Accordingly, there is provided a process capable of diminishing such anincrease in SO₂ emissions and maintaining effluent from the absorber ata continuous low level of emissions.

When ZnS is being regenerated to its active ZnO form, producing SO₂, theresulting regeneration effluent stream comprising SO₂ can be returned tothe Claus plant for removal of SO₂ by conversion to elemental sulfur.Reducing the volume of such a regeneration effluent stream can result inhighly significant cost savings in plant construction and retrofits aswell as in operation and maintenance.

There is provided a process which can reduce the volume of regenerationeffluent returned to a Claus plant and can achieve the significanteconomic benefits resulting therefrom.

SUMMARY OF THE INVENTION

The invention comprises a process for continuously removing sulfurcompounds, for example, H₂ S and SO₂, from a Claus plant tailgas to anextremely low level. In this process, the sulfur compounds are removedin the presence of an absorbent based on ZnO to produce a laden,sulfided absorbent (ZnS) and a purified gaseous stream (absorbereffluent). The volume of regeneration effluent produced duringregeneration of laden absorbent and returned to the Claus plant isreduced in accordance with the invention.

The invention comprises a new and advantageous combination of steps foruse in a sulfur recovery process. In the sulfur recovery process, a H₂ Scontaining stream is introduced into a Claus plant comprising a Clausthermal reaction zone (furnace) and at least one Claus catalyticreaction zone operated above the sulfur dewpoint (Claus high temperaturereactor). In the Claus plant, H₂ S is converted to elemental sulfur bythe Claus reaction and removed producing a Claus plant tailgas. The newand advantageous combination of steps reduces the volume of regenerationeffluent returned to the Claus plant while retaining the advantages ofusing absorber effluent for diluting O₂ during regeneration. The newcombination of steps comprises introducing the Claus plant tailgas intoat least one additional Claus catalytic reaction zone operated underconditions, including temperature, for depositing a preponderance ofsulfur on Claus catalyst therein ("Claus low temperature adsorptionzone"). The stream exiting such zone is then introduced into a firstabsorption zone containing ZnO absorbent, optionally after convertingsulfur species in such stream to H₂ S, and at least H₂ S is reacted withthe absorbent producing sulfided absorbent and absorber effluent.Concurrently, absorber effluent and O₂ are being introduced into thesecond absorption zone (functioning as a regenerator) containingsulfided absorbent for a regeneration period during which ZnS is beingconverted to ZnO. Regeneration effluent comprising SO₂ produced duringsuch conversion is returned to the Claus plant. The O₂ introduced duringthe regeneration period is an amount equal to about stoichiometricoxygen required for converting ZnS to ZnO (3/2 times the moles ofabsorbed sulfur atoms in the form of zinc sulfide), in addition, asappropriate, to about stoichiometric oxygen for combusting H₂ and CO inthat portion of absorber effluent used as diluent for regeneration towater and carbon dioxide. The rate of absorber effluent introductioninto the regenerator as diluent during regeneration is effective toreduce the total volume of regeneration effluent returned to the Clausplant during regeneration to a volume less than that returned in theabsence of the step of introducing the gas-in-process in the Claus plantinto at least one Claus low temperature adsorption zone. According toanother aspect of the invention, the absorber effluent is introduced asdiluent during regeneration at a rate less than about 35% of theabsorber effluent rate leaving the first absorption zone.

The invention accordingly comprises the processes and systems, togetherwith their steps, parts, and interrelationships which are exemplified inthe present disclosure, and the scope of which will be indicated in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a first embodiment of the invented process.

FIG. 2 shows schematically a second embodiment of the invented process.

FIG. 3 shows graphically an increase in SO₂ emissions occurring wherefreshly regenerated absorbent is not purged before absorption with aneffective reducing gas.

FIG. 4 shows graphically that such an increase in SO₂ emissions as shownin FIG. 3 can be eliminated by purging before absorption with aneffective reducing gas.

DETAILED DESCRIPTION OF THE INVENTION

Sulfur is recovered from an H₂ S-containing stream by introducing thestream into a Claus plant comprising a thermal reaction zone (Clausfurnace) and at least one Claus catalytic reaction zone. The Clausthermal reaction zone can be, for example, a Claus muffle tube furnace,a fire tube furnace, or the like. Generally, the Claus thermal reactionzone functions for converting a portion of H₂ S, preferably about 1/3,to SO₂ for thermal or catalytic Claus reaction with H₂ S to formelemental sulfur.

In the Claus furnace, the H₂ S-containing gas and oxidant can be reactedat a temperature generally in the range of about 1800°-2600° F. Theeffluent from the Claus thermal reaction zone can be cooled, forexample, in a waste heat boiler, and optionally passed through a sulfurcondenser to condense and remove liquid sulfur.

The gaseous effluent can then be fed into a Claus catalytic reactionzone operated above the sulfur dewpoint having an inlet temperature inthe range, for example, of about 350°-650° F. In the Claus hightemperature catalytic reactor, sulfur is formed by the Claus reaction(shown below) in the presence of an effective Claus reaction-promotingcatalyst such as alumina or bauxite:

    2H.sub.2 S+SO.sub.2 →3S+2H.sub.2 O

Gas containing elemental sulfur vapor can be continuously removed fromthe reactor and provided to a sulfur condenser where sulfur is condensedand removed as a liquid. Gaseous effluent from the sulfur condenser canbe reheated, if desired, and passed to further high temperature Clausreactors and associated sulfur condensers as is known in the art. Theeffluent gas from the final sulfur condenser is then the Claus planttailgas.

Preferably, the Claus plant tailgas is from a Claus plant which includesat least one Claus catalytic reactor operated under conditions,including temperature, effective for depositing a preponderance of theformed sulfur on Claus catalyst therein. Such a Claus low temperatureadsorption zone can be broadly operated in the range of from about 160°to about 330° F., preferably in the range of from about 260°-320° F.Where a Claus low temperature adsorption zone is used, it may or may notbe followed by a sulfur condenser. Thus, the adsorber effluent may bethe Claus plant tailgas.

The operation of such Claus plants having Claus furnaces, Claus hightemperature reactors, and Claus low temperature adsorption reactors iswell known in the art and need not be further described here. See, forexample, U.S. Pat. Nos. 4,044,114; 4,426,369; 4,430,317; 4,473,541;4,482,532; 4,483,844; 4,507,275; 4,508,698, and numerous others.

The tailgas from such Claus plants comprises H₂ S, SO₂, organicsulfides, and reducing species such as H₂ and CO. Tailgas from plantshaving only Claus high temperature reactors can contain H₂ S in therange of about 0.4 to about 4 mol. %, SO₂ in the range of about 0.2 toabout 2 mol. %, water in the range of about 20 to about 50 mol. %(typically 30-40 mol. %), as well as organic sulfides such as COS andCS₂, and elemental sulfur. Where the tailgas is from a plant having oneor more Claus low temperature adsorption reactors, the tailgas may haveequivalent of about 0.4 mol. %, preferably about 0.2 mol. %, or lesssingle sulfur species.

Use of at least one Claus low-temperature adsorption reactor ispreferable in part because such reactors remove significant amounts oforganic sulfides, such as COS, CS₂, and the like from the gas inprocess. These organic sulfides are not removed by sulfur recoveryprocesses such as the IFP process described in DeZael, et al., U. S.Pat. No. 4,044,114 (1977) which forms elemental sulfur in the presenceof polyethylene glycol and sodium benzoate. See, e.g., Kohl andRiesenfeld, Gas Purification, pages 491-493 (3d Ed. 1979).

For the same reason, it is also preferred to operate at least one Claushigh temperature reactor so that effluent has a temperature in the rangefrom about 550° to 700° F., preferably from about 600° to 650° F. todiminish the amount of organic sulfides in the effluent. See, e.g.,Kunkel, et al., U.S. Pat. No. 4,035,474 (1977).

Both H₂ S and SO₂, as well as organic sulfides, can be concurrentlyremoved in the absorber containing ZnO in the presence of reducingspecies for reducing the SO₂ and other sulfur species to H₂ S.Alternatively, sulfur containing species other than H₂ S can beconverted to H₂ S in a hydrogenation zone prior to introduction into theabsorber. In either case, it is preferred to operate the Claus plant sothat about a 2:1 ratio of H₂ S:SO₂ is maintained in the Claus planttailgas to maximize sulfur recovery in the Claus plant and to minimizethe amount of sulfur remaining in the Claus plant tailgas to be removedby the ZnO absorbers. Such ratio can be maintained by control systemswell known in the art and need not be further described here. Byreducing the organic sulfide and other sulfur content in the feed to theZnO absorbers, the volume of regeneration effluent returned to the Clausplant can be reduced or diminished. An effect of operating at about a2:1 ratio, however, is that quantities of both H₂ S and SO₂ are presentin the Claus plant tailgas, i.e., more than about 250 ppm of each of H₂S and SO₂.

The reducing species, for example, H₂ and/or CO required for conversionof sulfur compounds in the tailgas to H₂ S can be obtained from anyconvenient source including that present in the tailgas as H₂, oravailable from a donor such as CO, which can react with water to yieldH₂. H₂ is preferred, whether contained in the tailgas or internallygenerated or provided from an outside source.

The Claus plant tailgas can contain sufficient reducing species wherethe Claus plant is appropriately operated. For most Claus plants, byoperating the Claus furnace so that slightly less air is utilized thanthat required for reaction (1)

    H.sub.2 S+O.sub.2 →H.sub.2 O+1/2S+1/2SO.sub.2       (1)

and by insuring that the tailgas leaving the final sulfur condenser ofthe Claus plant has a low level of residual elemental sulfur, the Clausplant tailgas will contain sufficient reducing species. By furtherreducing the amount of oxidant introduced into the Claus furnace or byother methods which will be apparent to persons skilled in the art, theamount of reducing species can be further increased if desired.

The Claus plant tailgas having sufficient reducing species to reduce allsulfur compounds therein to H₂ S can be heated, for example, directly bymeans of direct fired heaters, or indirectly by heat exchange, forexample, with other process streams such as absorber effluent, toproduce a heated Claus plant tailgas effluent stream having atemperature effective for removal of both H₂ S and SO₂ in the presenceof a solid particulate preferably high surface area (for example,pellets, extrudates, and the like) ZnO absorbent effective for suchremoval. This simultaneous removal of both H₂ S and SO₂ is considered toproceed by hydrogenation of sulfur compounds present in the tailgas toH₂ S in the presence of ZnO, ZnO in this respect acting as a catalyst,followed by absorption of the thus-formed H₂ S by the ZnO by sulfidingthe ZnO to ZnS, the ZnO acting as an absorbent. Preferably, the Clausplant tailgas can be heated to above about 1000° F. As illustrated inEXAMPLE I below, by operating at these absorber temperatures, ahydrogenation reactor is not required before removal of sulfur compoundsother than H₂ S in the absorber. Conversely, temperatures below about1000° F. can be used during absorption with the addition of a separateand distinct hydrogenation reactor or zone prior to the absorbers. Whenoperating at temperatures above about 1000° F., H₂ S emissions and thereduction of ZnO to Zn vapor under a reducing environment can set apractical upper limit on the absorption temperature which will be used.Currently for these reasons it may be appropriate that the upper limitduring absorption be about 1200° F. Higher temperatures can also beused. Absorber operation above about 1000° F. is preferred because suchhigher temperatures favor higher absorption capacity and thehydrogenation reactor can be eliminated. Also, since absorption andregeneration will then be conducted at approximately the same inlettemperature (1000°-1200° F.), temperature stress on equipment can bereduced. As a result, there will be no significant heating and coolingperiods. Hence, the time available for regeneration will be increasedand the rate of regeneration effluent returned to the Claus plant can bedecreased.

Where the Claus plant is introduced into a hydrogenation zone prior tothe ZnO absorbers, the principal reaction will be the conversion of SO₂to H₂ S as shown by Reaction (6) below; other sulfur compounds includingelemental sulfur, COS, CS₂, and the like will also be reduced to H₂ S.Hydrogenation can be carried out at a temperature of from about 450° toabout 1200° F. or even higher, preferably from about 580° F. to about900° F., depending on the conditions and the source of H₂ chosen.Hydrogenation by contacting with a bed, either supported or fluidized,of effective hydrogenation catalyst is preferred. Useful catalysts arethose containing metals of Groups VB, VIB, VIII and the Rare Earthseries of the "Periodic Table of the Elements" in Perry and Chilton,Chemical Engineers Handbook, 5th Ed. The hydrogenation catalysts may besupported or unsupported. Catalysts supported on a refractory inorganicoxide, such as on a silica, alumina or silica-alumina base arepreferred. The preferred catalysts are those containing one or more ofthe metals, cobalt, molybdenum, iron, chromium, vanadium, thorium,nickel, tungsten (W) and uranium (U) added as an oxide or sulfide of themetal, although the sulfide form appears to be the active form.Particularly preferred are cobalt-molybdenum hydrogenation catalystssuch as are commercially available for use in the refining industry fordesulfurization processes in the refining of oil.

After hydrogenation, the resulting stream now containing substantiallyall sulfur compounds in the form of H₂ S can then be contacted in anabsorber zone with a suitable ZnO absorbent (either fixed or fluidizedbed) to absorb H₂ S and to produce a laden (sulfided) absorbent attemperatures in the range of about 600° F. to about 1000° F.Alternatively, where absorption is conducted at a temperature aboveabout 1000° F., for example, in the range of about 1000° F. to about1200° F., the absorption of H₂ S can be accomplished in an absorbersimultaneously with removal of the other sulfur compounds without priorhydrogenation. In either event, while a first absorption zone isfunctioning as an absorber, a second absorption zone can be functioningas a regenerator.

As used herein, and in the claims, the terms "absorbent", "ZnO", "ZnOabsorbent", and the like shall mean an absorbent effective for removalof both H₂ S and SO₂ in the presence of reducing species. A majorportion of the active absorbent, for example, fifty percent or more, isin the form of ZnO which is the active form. The absorbent can alsocontain binders, strengtheners, and support materials, for example,alumina (Al₂ O₃), calcium oxide (CaO) and the like. Zinc sulfide andzinc sulfate can be used as starting materials and treated with heatand/or oxygen to produce an active ZnO sorbent. Other suitable startingmaterials can also be used. The ZnO absorbent is effective for absorbingH₂ S by undergoing sulfidization to produce a laden (sulfided)absorbent; simultaneously, if desired, hydrogenation of other sulfurcompounds to H₂ S followed by such absorption can occur. Preferably, theZnO absorbent is capable of a high level of removal of sulfur compoundsand is relatively insensitive to water.

Particularly preferred are ZnO absorbents which are thermally stable,regenerable, and capable of absorbing substantial amounts of sulfurcompounds. An acceptable absorbent is United Catalysts, Inc., G72DSulfur Removal Catalyst, available from United Catalysts, Inc.,Louisville, KY, having the following chemical composition and physicalproperties:

    ______________________________________                                        CHEMICAL COMPOSITION                                                          wt %          Trace Metal Impurities                                                                          wt %                                          ______________________________________                                        ZnO     90.0 ± 5%                                                                            Pb                <0.15                                     Carbon  <0.20     Sn                <0.005                                    Sulfur  <0.15     As                <0.005                                    Chlorides                                                                             <0.02     Hg                <0.005                                    Al.sub.2 O.sub.3                                                                      3-7       Fe                <0.1                                      CaO     0.5-3.0   Cd                <0.005                                    ______________________________________                                    

    ______________________________________                                        PHYSICAL PROPERTIES                                                           ______________________________________                                        Form             Pellets                                                      Size             3/16 in.                                                     Bulk Density     65 ± 5 lbs/ft3                                            Surface Area     35 m2/g minimum                                              Pore Volume      0.25-0.35 cc/g                                               Crush Strength   15 lbs minimum average                                       ______________________________________                                    

Representative chemical reactions considered to occur during absorption,regeneration and purging are shown below:

During Absorption:

    H.sub.2 S+ZnO→ZnS+H.sub.2 O                         (3)

    COS+ZnO→ZnS+CO.sub.2                                (4)

    CS.sub.2 +2ZnO→2ZnS+CO.sub.2                        (5)

    SO.sub.2 +3H.sub.2 →H.sub.2 S+2H.sub.2 O            (6)

    H.sub.2 S+Sulfated Absorbent→SO.sub.2 +ZnO Absorbent(7)

During absorption, H₂ S, COS and CS₂ in the stream can react with ZnO toform ZnS as shown in Eqs. (3) to (5). SO₂ can react directly with H₂ toform H₂ S as shown by Eq. (6), and the resulting H₂ S can then reactwith ZnO. COS and CS₂ may also be hydrogenated and/or hydrolysed to H₂ Sbefore absorption by ZnO. When elements in the absorbent such as zinc,calcium, aluminum, or other elements become sulfated duringregeneration, SO₂ may be produced during absorption as indicated by Eq.(7) due to the presence of effective reducing species in the absorberfeed. Sulfation is reversed by purging the regenerated absorbent witheffective reducing species before returning regenerated ZnO toabsorption and returning the produced SO₂ to the Claus plant for sulfurformation and removal.

During Regeneration:

    ZnS+3/2O.sub.2 →ZnO+SO.sub.2                        (8)

    Absorbent+SO.sub.2 +O.sub.2 →Sulfated Absorbent     (9)

Regeneration of sulfided absorbent is effected by oxidizing ZnS to ZnOas shown by Eq. (8). Absorbent sulfation can also occur, as shown by Eq.(9) during regeneration in the presence of O₂ and SO₂. Temperature riseduring regeneration can suffice if unchecked to destroy both thephysical integrity and the chemical activity of the absorbent as well asto exceed metallurgical limits of preferred materials of construction.Consequently, temperature rise during regeneration is preferablycontrolled to less than about 1500° F.

During Purging:

    Sulfated Absorbent+H.sub.2 →Absorbent+SO.sub.2 +H.sub.2 O(10)

    Sulfated Absorbent+CO→Absorbent+SO.sub.2 +CO.sub.2  (11)

Reduction of the sulfated absorbent will occur at temperatures aboveabout 1000° F. in the presence of H₂, CO or other reducing species suchas H₂ S. Reduction of the sulfated absorbent is not effected at lowertemperatures such as 900° F. or lower or in the absence of sucheffective reducing species.

Methane is not effective in reasonable periods of time under processconditions for purging in accordance with the invention. Further,purging with an inert gas will not prevent the SO₂ emissions increaseupon returning to absorption. Rather, upon switching to absorption, thesulfated ZnO absorbent will be contacted with a stream containing theeffective reducing species (H₂, CO, and H₂ S) and SO₂ emissions willoccur. Accordingly, for the purging, it is essential that effectivereducing species be present and that the temperature be greater thanabout 1000° F., but preferably not much greater than about 1200° F.since significant losses of zinc can occur above that temperature in thepresence of reducing species. Nevertheless, higher temperatures can beused.

The absorber zone containing ZnO can comprise at least a firstabsorption zone (functioning as an absorber) and a second absorptionzone (functioning as regenerator) and the process can comprisecontacting H₂ S with absorbent in the absorber to remove it and othersulfur species producing a laden absorbent and absorber effluent lean insulfur species. Absorption can be continued for a period of time(absorption period), preferably less than that required for H₂ Sbreakthrough from the absorber. For practical purposes, H₂ Sbreakthrough can be defined as occurring when the H₂ S concentration inthe absorber effluent stream reaches a preset low value, such as forexample, 50 ppm H₂ S. As shown in EXAMPLES I and II below, breakthroughtime and absorption capacity increase with increasing absorbertemperature. Concurrently with absorption in the absorber, ladenabsorbent in the regenerator can be regenerated by introducing aregeneration stream comprising dilute O₂ thereinto at a temperatureeffective for converting laden sulfided absorbent to active absorbent.Regeneration effluent comprising SO₂ is returned from the regenerator tothe Claus plant, for example, to the thermal reaction zone or to adownstream Claus catalytic reaction zone. Thereafter, the absorber andthe regenerator can be interchanged, with the second absorption zone nowfunctioning as absorber and the first absorption zone now functioning asregenerator, and the process can be repeated and continued. Prior tointerchanging the absorber and the regenerator, freshly regeneratedabsorbent in the regenerator is purged with an effective reducing gas.

During regeneration, a temperature rise of about 145° F. occurs for eachmol percent of oxygen consumed in converting ZnS back to ZnO. To avoidexceeding metallurgical limits and to maintain absorbent physical andchemical integrity during regeneration, a maximum of about 3.5 mol. %oxygen can be used during regeneration when the regeneration stream isintroduced at about 1000° F., and a maximum of about 2.75 mol. % O₂ whenthe regeneration stream is introduced at about 1100° F. Thus, preferablyoxygen is introduced during regeneration at a concentration of about 0.4or less to about 3.5 mol. %, more preferably at about 1 to about 2.75mol. %. Due to the exothermic nature of the regeneration reaction,suitable methods for diluting the oxygen can be used. Wheremetallurgical limits are not controlling, regeneration can occur atmaximum temperatures as high as about 2100° F.

Prior to the discoveries set forth herein, suitable methods for dilutingthe oxygen would have have been considered to include the following: (1)a portion of the regenerator effluent can be recycled back to theregenerator to dilute O₂ in the regeneration stream; (2) a portion ofabsorber effluent can be used to dilute O₂ in the regeneration stream.

When method (1) is employed, the SO₂ level during regeneration in theregenerator is higher than when method (2) is used since SO₂ producedduring regeneration is recycled to the regenerator. Reference to EXAMPLEVI indicates that higher SO₂ levels during regeneration favors sulfationof the absorbent. It has also been found when method (1) is used thatSO₂ emissions are larger upon returning to absorption, and/or that alonger purge time is required prior to returning to absorption toeliminate an increase in SO₂ emissions following return to absorption.Accordingly, method (2) is preferred for diluting the O₂ to a suitableconcentration for regeneration.

The flow rate during regeneration is preferably a rate sufficient tocomplete regeneration and purging as described herein of a ZnO absorberduring effective absorption in another ZnO absorber. In this way, onlytwo absorption zones will be required. Some time can also be allowed forthe contingency of process upsets (slack time). Preferably, the flowrate during regeneration is such that the period during whichregeneration is occurring is equal to the period during which absorptionis occurring less the period required for purging as herein set forthand such slack time.

As indicated, regeneration effluent comprising SO₂ is returned from theregenerator to the Claus plant for conversion of the SO₂ to elementalsulfur which is removed from the gas in process. Dilution of the O₂using absorber effluent minimizes purge time and/or magnitude of the SO₂emissions at the start of absorption but can result in a large volume ofregeneration effluent being returned to the Claus plant. This has theundesired result of increasing the size of the Claus plant and equipmentdownstream of the locus where the regeneration effluent is reintroducedresulting in significant cost increases.

The rate of regenerator effluent returned to the Claus plant could bereduced by recycling regenerator effluent as diluent back to theregenerator. However, this results in larger SO₂ emissions at the startof absorption and/or longer purge time prior to absorption. Othermethods which might be used to reduce the volume of regenerator effluentare also disadvantageous. As an example, O₂ concentration could beincreased during regeneration resulting in a smaller volume ofregeneration effluent. However, O₂ concentration in excess will causetemperature rise during regeneration to exceed metallurgical limits ofconvenient materials of construction and can result in damage toequipment or to the absorbent. As another example, more than two ZnOabsorbers might be used, so that while one absorber is on absorption,two or more other absorbers could be in various stages of regenerationand purging. This, however, also increases cost.

Absorber effluent can be used for diluent and the rate of regenerationeffluent returned to the Claus plant can still be reduced (1) where theClaus plant comprises at least one Claus low-temperature adsorptionreactor, (2) where O₂ introduced during the regeneration period when ZnSis being converted to ZnO is in an amount about equal to thestoichiometric amount for such conversion, that is, about 3/2 moles O₂for each mole of ZnS to be regenerated, and (3) where the rate ofabsorber effluent diluent introduced during the regeneration is suchthat the rate of regeneration effluent during the regeneration period isless than the rate of regeneration effluent returned in the absence oftreating in a Claus low-temperature adsorption reactor prior totreatment in a ZnO-containing absorber. As absorber effluent typicallycomprises residual H₂ and CO, the amount of O₂ introduced can furtherinclude about the stoichiometric amount required for combusting H₂ andCO to water and CO₂.

By use of a low-temperature Claus adsorption reactor, the absorptionrate for a ZnO absorber is decreased since the sulfur content of thefeed to the absorber is reduced, allowing O₂ to be introduced to theregenerator at a lower rate. By introducing O₂ during the regenerationperiod in a total amount effective for oxidizing ZnS to ZnO and, asappropriate, also for combusting any residual H₂ and CO to H₂ O and CO₂,the total volume of O₂ is minimized. This permits the rate of absorbereffluent introduced as diluent into the regenerator during regenerationto be such that the volume of regeneration effluent returned to theClaus plant during regeneration can be reduced in comparison with thevolume where a Claus low-temperature adsorption reactor is not used.This result can be achieved by controlling the rate at which absorbereffluent is introduced into the regenerator as diluent, so that thevolume of regeneration effluent returned to the Claus plant is reducedin comparison with the volume where a Claus low-temperature adsorptionreactor is not used while still maintaining, for example, thetemperature in the regenerator at a desired level, i.e., below about1500° F. Preferably, the absorber effluent is introduced into thereactor being regenerated at a rate less than about 35% of the absorbereffluent rate leaving the first absorption zone. Such rates of less than35% have been found to be effective to reduce the size of the Clausplants upstream of the ZnO absorbers as compared with Claus plantslacking Claus low temperature adsorption zones. More preferably, theabsorber effluent is introduced as diluent into the regenerator at arate less than about 20% of the absorber effluent rate to allow a largemargin to compensate for process upsets, slack time followingregeneration and purging, and the like and still reduce the volume ofregeneration effluent recycled to the Claus plant. Most preferably, theabsorber effluent is introduced into the regenerator at a rate less thanabout 10% of the absorber effluent rate since such absorber effluentdiluent rates have been found to greatly reduce the volume ofregenerator effluent returned the Claus plant when a Claus lowtemperature adsorption reactor is used in accordance with the invention.As discussed below in EXAMPLE XII, this combination of stepssignificantly reduces the size of the plant required upstream of the ZnOabsorbers and has the unexpected results that plants in accordance withthe invention having more equipment in terms of vessels, condensers, andthe like can cost less than a plant having less equipment for processingthe same gas stream.

Regeneration can be preferably continued until substantially all of thesulfided absorbent is regenerated, for example, until ZnS issubstantially reconverted to ZnO. Completion of regeneration can beconveniently determined by monitoring O₂ or SO₂ content or temperatureof the regenerator effluent stream. Preferably, an O₂ analyzer isemployed downstream of the regenerator to determine the presence of O₂in the regenerator effluent, which is an indication of completion ofregeneration.

As will be appreciated by those skilled in the art from the foregoingdiscussion, materials of construction for the valves, vessels, andpiping for the process according to the invention can require specialattention. The material preferably has the capability of withstandinghigh temperatures, for example, in the range of about 800° F. to about1500° F. or higher while being repeatedly exposed to reducing andoxidizing atmospheres in the presence of sulfur compounds.

Following regeneration, prior to returning the regenerated absorbent foruse during the absorption cycle, the regenerated absorbent is treated(purged) by passing a reducing stream in contact with the regenerated,albeit sulfated absorbent (see Examples VI-VII), for a period of timeeffective for reducing by at least 10% a temporary increase in SO₂emissions otherwise occurring when freshly regenerated ZnO absorbent isreturned to absorption without such purging with a reducing gas.Preferably, the time is effective for reducing SO₂ emissions to belowabout 250 ppm at all times. Most preferably, the time is effective forsubstantially eliminating the increasing SO₂ emissions, that is, forreducing the increase in SO₂ emissions above the usual level duringabsorption by 90% or more from the level occurring where such a reducinggas purge is not used prior to returning to absorption.

The effective purge time can be readily determined by one skilled in theart by monitoring SO₂ emissions from an absorber following returning afreshly regenerated reactor to absorption function and increasing thepurge time for a given reducing gas stream prior to returning toabsorption until the SO₂ emissions are reduced to a desired level uponreturning to absorption. In using the absorber effluent for purging, apurge time in the range of about 1/2 to about 3 hrs can be effective,preferably, in the range of about 1 to about 2 hrs. The stream used forpurging can be any reducing stream containing reducing species such asH₂, CO or H₂ S. Preferably H₂ or CO are used since these do not resultin loading the absorbent during purging. The reducing species shouldpreferably be present in an amount effective for reducing the sulfatedabsorbent in the purge period. For example, at a space velocity of 1lb-mol/hour/cu ft of absorbent, for a purge period of about 1/2 to about3 hrs, the reducing species can be present in the range of about 6.4 toabout 1.1 mol. %; similarly, for a reducing period in the range of about1 to about 2 hrs, the reducing species can be present in an amount ofabout 3.2 to 1.6 mol. %. Other space velocities, reducing speciesconcentrations, and the like can be readily determined by those skilledin the art.

Preferably, the purge stream can comprise at least a portion of absorbereffluent. Most preferably, the purge can be effected by using the sameportion of absorber effluent used for regeneration, by discontinuing theflow of O₂ to the regenerator during the purge period.

The invention will be further understood by the EXAMPLES which are setforth below.

EXAMPLE I Absorption: Effect of Temperature

The effect of temperature on H₂ S breakthrough is studied using alaboratory catalyst holder/reactor made from type 304 stainless steeltubing 2" (inch) diameter (O.D.)×0.068" thick wall, 27" long overall.Calculated catalyst volume for 18" depth is 805 ml (milliliters), andthe catalyst is supported by a 20 mesh stainless steel screen. Catalystused is G72D Sulfur Removal Catalyst described above. The reactor iswrapped by six heaters (22 gauge nichrome wire) for preventing radialheat loss, and is insulated with fiberglass. The total flow rate forabsorption is 10 l./min (liters/min) and for regeneration 5 l./min. Thereactor is placed in a large Blue M® oven, available from Blue MElectric Company, Blue Island, IL. All gas flow through the catalyst bedis downflow. Provisions for side draw of gas samples are available nearthe reactor axis each 1.5" of catalyst depth.

The effect of reaction temperatures on H₂ S breakthrough time duringabsorption is illustrated by introducing a feed gas having the followingcomposition into the reactor inlet:

    ______________________________________                                               H.sub.2 S   0.8    mol %                                                      SO.sub.2    0.4    mol %                                                      CO          1.0    mol %                                                      H.sub.2 O   30.0   mol %                                                      N.sub.2     45.8   mol %                                                      H.sub.2     2.0    mol %                                                      CO.sub.2    20.0   mol %                                               ______________________________________                                    

The feed gas is introduced at 850° F., at 1000° F., and at 1150° F.Breakthrough, defined for purposes of these runs as 50 ppm H₂ S in theabsorber effluent, and H₂ S concentration in the effluent gas atequilibrium, are determined. Results are set forth in the followingTable IA.

                                      TABLE 1A                                    __________________________________________________________________________                    Combined SO.sub.2 and                                                         H.sub.2 S Concentration                                                                 Absorption Capacity                                         Time (Hrs) for                                                                        (Dry Basis)   mols absorbed/                                  Run                                                                              Temp.                                                                              Breakthrough                                                                          at Equilibrium                                                                          wt %                                                                              mols sorbent                                    __________________________________________________________________________    1   850° F.                                                                    (Immed. SO.sub.2                                                                      --        --  --                                                      Breakthrough)                                                         2  1000° F.                                                                    25.5 hrs                                                                              <10 ppm   33% 0.84                                            3  1150° F.                                                                    27.5 hrs                                                                              <20 ppm   36% 0.92                                            __________________________________________________________________________

The results indicate that higher temperatures favor increased absorptioncapacity as indicated by increased breakthrough times and that lowertemperatures favor lower equilibrium concentrations of H₂ S in theabsorber effluent streams. It is also noted that at 1000° F. and at1150° F., SO₂ present in the inlet stream is substantially completelyabsorbed; while at 850° F., SO₂ appears immediately in the absorbereffluent stream. Thus at temperature at least about 1000° F. and higherhydrogenation of SO₂ to H₂ S is not required prior to absorption.

EXAMPLE II Absorption: Effect of Temperature

The effect of temperature on H₂ S breakthrough is further investigatedby the following runs using the apparatus described in EXAMPLE I andusing an inlet stream having the following composition:

    ______________________________________                                               H.sub.2 S   1.2    mol %                                                      H.sub.2 O   29.5   mol %                                                      H           1.06   mol %                                                      CO          1.01   mol %                                                      CO.sub.2    20.39  mol %                                                      N.sub.2     46.83  mol %                                               ______________________________________                                    

This inlet stream can be used to simulate the condition where SO₂present in a Claus plant effluent stream is hydrogenated to H₂ S priorto absorption. Breakthrough time for various temperatures below 850° F.are determined and are shown in Table IIA below:

                  TABLE IIA                                                       ______________________________________                                                Time (Hrs) for                                                                            Absorption Capacity                                       Run   Temp.   Breakthrough  wt %  mols/mol sorbent                            ______________________________________                                        4     625° F.                                                                         3             4%   0.10                                        5     700° F.                                                                        11            14%   0.36                                        6     775° F.                                                                        17            22%   0.46                                        ______________________________________                                    

These results further confirm the dependence of absorption capacity andbreakthrough on absorption temperature.

EXAMPLE III Absorption: Effect of Water

The effect of the presence of water on sulfur compound breakthrough isillustrated in part by EXAMPLE I above in which a feed gas streamcontaining 30.0% water is contacted with a ZnO absorbent and, at 1000°F. to 1150° F., the sulfur compounds in the effluent stream are reducedto 20 ppm or lower.

To further investigate the effect of water on sulfur compoundbreakthrough using a metal oxide absorbent, the apparatus of EXAMPLE Ican be used with a zinc ferrite absorbent containing about 45% ironoxide and about 55% amorphous silica. About 15% of the 45% iron oxide isin the form of zinc ferrite. A feed gas having the following compositionis introduced into the reactor inlet at 1000° F:

    ______________________________________                                               H.sub.2 S     1.2%                                                            CO            1%                                                              H.sub.2       2%                                                              CO.sub.2.sup.1                                                                             20% (42%)                                                        H.sub.2 O.sup.1                                                                            22% (0%)                                                         N.sub.2      53.8%                                                     ______________________________________                                         .sup.1 CO.sub.2 content of inlet stream is increased from 20% to 42% when     22% H.sub.2 O is eliminated from the feedstream.                         

After about 51/2hrs, water is eliminated from the feedstream. Theresults are shown in Table IIIA below.

                  TABLE IIIA                                                      ______________________________________                                        Time        H.sub.2 S Concentration                                           (Hrs)       in Reactor Effluent                                               ______________________________________                                        1           663                                                               2.3         733                                                               3.4         818                                                               4.1         994                                                               5.5.sup.1   1682                                                              5.7          9                                                                7.1          9                                                                8.6          9                                                                ______________________________________                                         .sup.1 Water eliminated from feedstream.                                 

The results indicate that the iron oxide (zinc ferrite) absorbent issensitive to the presence of water in the feedstream as compared withthe ZnO of EXAMPLE I. After water is removed from the feedstream, H₂ Sin the effluent stream is reduced to 9 ppm. These results indicate thatZnO is less sensitive to water than is iron oxide (zinc ferrite).

EXAMPLE IV Regeneration

Regeneration is investigated using the apparatus described in EXAMPLE Iby passing a dilute air stream in contact with the sulfided absorbent.The effect of temperature on regeneration is investigated. For a diluteair regeneration stream containing about 5 mol. % oxygen having an inlettemperature of about 1000° F., the sulfur recovered as SO₂ in theregeneration effluent stream is only 0.75 mol. %. However, when theinlet temperature is raised to 1150° F. after 51/2 hrs, about 3 mol. %of sulfur as SO₂ appears in the regeneration effluent stream. Thishigher regeneration temperature is considered preferred to overcome thehigh activation energy required for Reaction (8) above. Duringregeneration, the concentration of SO₂ in the regeneration effluentstream remains above about 3.5 mol. % and the concentration of O₂ in theregeneration effluent stream remains about 0 mol. %, indicatingsubstantially complete consumption of O₂, for about 22 hrs. After about22 hrs, when regeneration is about complete, O₂ starts to breakthroughand SO₂ content begins to decline in the regeneration effluent stream.

EXAMPLE V Effect of Purge

Effluent tailgas from a Claus sulfur recovery plant having two catalyticreactors operated above the sulfur dewpoint and one Claus lowtemperature adsorption reactor on-stream at all times is provided to anabsorber containing ZnO. A portion of absorber effluent is used as adiluent for O₂ to a regenerator containing ZnS. In a first run, uponcompletion of regeneration, the regenerator and absorber areinterchanged in function. Upon interchanging the absorbers, an emissionslevel from the freshly regenerated catalyst, now functioning as anabsorber, of about 350 ppm SO₂ is observed. SO₂ emissions decline toless than about 50 ppm in about two (2) hours. See FIG. 3. In a secondrun, upon completion of regeneration and prior to interchanging theabsorber and the regenerator, O₂ flow into the regenerator isdiscontinued and the flow of absorber effluent is continued for a periodof about two (2) hours. Upon interchanging the absorber and regenerator,SO₂ emissions from the absorber are initially less than about 50 ppm andcontinue at that low level. See FIG. 4. This example indicates thatdiscontinuing O₂ flow and continuing absorber effluent, or otherreducing gas flow, prior to interchanging an absorber and a regeneratoreliminates a temporary increase in SO₂ emissions above a baseline levelotherwise observed from the absorber after interchanging the tworeactors.

EXAMPLE VI Effect of Regeneration Gas Composition on Purge

The effect of SO₂ levels during regeneration upon purge timerequirements at the end of regeneration is investigated by regeneratingsulfided absorbent using regeneration feedstreams having various SO₂levels followed by purging with a reducing gas stream having 1.1% H₂ and0.5% CO at a space velocity of about 1. The results are set forth in thefollowing table:

    ______________________________________                                        Run     SO.sub.2 in Regeneration Feed                                                                  Purge Time (Hours)                                   ______________________________________                                        1       0%               2.0                                                  2        2.9%            4.5                                                  3       13.2%            >12                                                  ______________________________________                                    

The results indicate that the SO₂ level in the regeneration feed greatlyaffects the purge time and that increased levels of SO₂ duringregeneration increase the purge time requirements. The results indicatethat the use of absorber effluent or other reducing gas having little orno SO₂ present at the inlet is advantageous in reducing purge time.

EXAMPLE VII Effect of Regeneration Temperature on Purging/SubsequentAbsorption

Purging runs are made after regeneration at 900° F. and 1150° F. usingabsorber effluent as the purge gas. The test results show that bypurging at 900° F., the increase in SO₂ emissions is not removed,whereas by purging at 1150° F., increased SO₂ emissions were notobserved upon returning to absorption. Based upon these results, it isconsidered that purging should occur at temperatures from about 1000° F.to about 1200° F. consistent with the temperatures required forhydrogenation of other species in the presence of ZnO absorbent as setforth in Example I above.

EXAMPLE VIII Effect of H₂ on SO₂ Emissions

To investigate the effect of H₂ on SO₂ emissions, laden ZnO (ZnS) isregenerated at 1150° F. with a regeneration stream having the followinginlet composition:

                  TABLE IXA                                                       ______________________________________                                               O.sub.2     5      mol %                                                      NH.sub.3    720    ppm                                                        CO.sub.2    85     mol %                                                      H.sub.2 O   10     mol %                                               ______________________________________                                    

After SO₂ emissions decreased to about 50 ppm, 1 mol. % H₂ was added.SO₂ emissions immediately increased to about 450 ppm and then decreasedwith time. (Note: the NH₃ was present to simulate refinery gas in thisrun; however, the presence of NH₃ is not considered to affect theresults from the addition of H.sub. 2 reported herein.)

These results indicate that reducing equivalents such as H₂ result inSO₂ emissions from a freshly regenerated absorbent. Thus, these resultsindicate that the effect of reducing gases during the purge period is tocause the production of and allow the removal of SO₂ from regeneratedsulfated absorbent in the purge effluent stream prior to return toabsorption. SO₂ removed during purge in regeneration effluent is sent tothe Claus plant where sulfur is formed and removed from the process. Inthis way, SO₂ emissions from regenerated absorbent will not appear asemissions from the plant.

EXAMPLE IX Effect of Hydrogen Sulfide on Reducing SO₂ Emissions

The effect of H₂ S on reducing SO₂ emissions is investigated bycontacting freshly regenerated absorbent with a stream containing H₂ Sbut no SO₂. An SO₂ emissions peak of about 100 ppm is observedinitially, diminishing to about 20 ppm after six (6) hours. Theseresults indicate that H₂ S will be effective as a purge gas. It is notedthat H₂ S will also result in absorbent loading. See Eq. (3).

EXAMPLE X Effect of Methane on Reducing SO₂ Emissions

The effect of methane on reducing SO₂ emissions is investigated bycontacting absorbent, freshly regenerated with a stream comprising about13% SO₂, with methane for six (6) hours. At the end of the six (6)hours, SO₂ emissions are about 2000 ppm. Upon switching to absorption,with a stream comprising 0.39 mol. % H₂ S, 0.16 mol. % SO₂, 1.69 mol. %H₂, and 0.26 mol. % CO, SO₂ emissions of about 8000 ppm are observedwhich decrease to about 1000 ppm in about 7 hours. Mass spectrographicanalysis of the effluent stream during purge with methane indicates thatmethane is not cracked to H₂ and CO at regeneration temperatures ofabout 1100° F. These results indicate that methane alone is relativelyineffective for purging to reduce SO₂ emissions under processconditions.

EXAMPLE XI Analysis of Sulfided Absorbent

Samples of fresh absorbent and regenerated absorbent, regenerationhaving been conducted at 1150° F. in the presence of oxygen and 13% SO₂are analyzed by X-ray diffraction. The fresh absorbent is largelycrystalline ZnO (zincite). The regenerated absorbent contains ZnO as themajor component, with minor concentrations of zinc oxide sulfate Zn₃ O(SO₄)₂, anhydrite CaSO₄, and gahnite, ZnAl₂ O₄. These results indicatethat sulfated compounds may be the cause of SO₂ emissions when reducedby contacting with a reducing gas stream.

EXAMPLE XII Effect of Low Sulfur Content and Low Absorber Diluent Rate

The effect of reducing the volume of regeneration effluent returned to aClaus plant having absorbers for tail gas cleanup is investigated bymodeling and comparing two process configurations. The firstconfiguration is a three reactor Claus plant containing a furnace, atwo-pass waste heat boiler, three Claus reactors, and four sulfurcondensers. All of the Claus reactors are operated above the sulfurdewpoint. The Configuration 1 plant is capable of achieving an overallsulfur recovery of about 97.4% when there is no regeneration effluentstream introduced from the absorbent regenerator. The secondconfiguration contains a furnace, a two-pass waste heat boiler, twoClaus catalytic reactors operated above the sulfur dewpoint, and twoadditional Claus reactors, at least one of which, and for short periods,both of which are operated under low-temperature Claus adsorptionconditions, and four sulfur condensers. The Configuration 2 plant iscapable of an overall sulfur recovery of about 99.4%. For eachconfiguration, the acid gas feed rate and composition to the plant isthe same, and each configuration is modeled for two ZnO absorptionzones, one functioning as an absorber, and one functioning as aregenerator. For each configuration, the Claus plant portion of theconfiguration, including Claus low-temperature adsorption reactors, ismodelled using a sulfur plant design program and the zinc oxideabsorbers are modelled by calculation from experimental laboratory dataverified from pilot plant data. In both configurations, the O₂concentration in the regenerator feed is such that the temperature inthe regenerator is maintained at about 1450° F. when the temperature ofthe feed to the regenerator is about 1100° F. 1450° F. is selected as areasonable practical approach to about 1500° F., which is preferred forachieving economic advantage of nonrefractory lined materials ofconstruction.

The results are shown below.

    ______________________________________                                                           Config. 1                                                                             Config. 2                                          ______________________________________                                        Sulfur Content of Absorber Feed                                               Total molar flow rate of all                                                                        0.71%     0.17%                                         sulfur compounds as Sl                                                        divided by total Claus                                                        tailgas molar flow rate:                                                      Percent of preceding 100%      24%                                            Configuration 1 value:                                                        Regeneration Effluent Rate                                                    Percent of acid gas flow                                                                           163%      28%                                            rate to Claus plant                                                           furnace:                                                                      Percent of preceding 100%      17%                                            Configuration 1 value:                                                        Percent of the Claus tail-                                                                          43%      11%                                            gas molar flow rate:                                                          Percent of preceding 100%      26%                                            Configuration 1 value:                                                        Absorber Effluent Diluent Rate                                                Percent of absorber effluent                                                                        33%       8.5%                                          rate:                                                                         ______________________________________                                    

The table indicates that the volume of regeneration effluent returned tothe Claus plant (and therefore the incremental increase in the size ofthe Claus plant due to the regeneration effluent stream which isrecycled to it since Claus plants are sized on volume of gas processed)expressed as a function of the acid gas rate is reduced to a lower value(17%) from what would have been predicted from the reduced sulfurcontent of the gas being treated in the ZnO absorber zones alone (24%).This is because of the compound effect of the reduced sulfur content ofthe gas being treated in the ZnO absorber zones and the reduced amountof absorber effluent (diluent) required to moderate the oxygenconcentration in the zone being regenerated (ZnO absorbent beingconverted from ZnS to ZnO). Reducing the sulfur content of the gas beingtreated in the ZnO absorber by using a Claus low-temperature adsorptionzone reduces the rate at which the ZnO absorbent is converted from ZnOto ZnS. Therefore, the rate at which ZnS can be regenerated back to ZnOis also reduced which reduces the rate of oxygen fed to the absorptionzone during regeneration. The rate of diluent required for theregeneration feed gas is also reduced in order to maintain the sameoxygen concentration. Reducing the regeneration rate (and diluent rate)reduces the regenerator effluent recycle rate. Reducing the recycle ratereduces the total Claus plant feed and thus the rate of the Claustailgas which is fed to the ZnO absorber. A reduced tailgas rate againreduces the rate of sulfur compounds flowing into the ZnO absorber,which again reduces the absorption rate, regeneration rate, diluentrate, and recycle rate.

Comparing the regeneration effluent rate from the regenerator with thefeed rate to the ZnO absorber for each of the configurations, thereduction in the regeneration effluent rate (to 26%) betweenConfiguration 2 and Configuration 1 is approximately the same as thereduction in the content of sulfur in the ZnO absorber feed (to 24%),allowing for rounding differences and slight differences in calculation.This indicates that the reduction in the regeneration effluent to 17% ascompared with the acid gas rate is due to the reduced regeneratoreffluent recycle caused by the lower sulfur content of the absorber feedcombined with the reduced fraction of absorber effluent used as diluent.

In Configuration 2 above, the diluent added to the air used for theregeneration of the ZnO absorber could be increased from 8.5% to as highas about 35% of the absorber effluent rate while maintaining a lowerrate of regeneration effluent than exists for Configuration 1. However,the diluent for Configuration 1 cannot be decreased below about 33% ofthe absorber effluent without suffering one or both of the followingconsequences: (1) if the air is held constant, then decreasing thediluent rate increases the oxygen concentration in the regeneration feedstream and the maximum temperature in the absorber being regenerated canbecome high enough to exceed a desired limit (e.g., 1500° F.), and (2)if the air rate is decreased proportional to the diluent rate decrease,then the absorbent will not all be regenerated in the time allowed forregeneration, resulting in less active absorbent available for theabsorption period, which will result in either a shortened absorptionperiod (and regeneration period for the other absorber, therebyincreasing the amount of absorbent not properly regenerated) or a periodof high sulfur emissions after all of the ZnO in the absorption zone hasbeen converted to ZnS.

Higher maximum temperatures, up to about 2100° F., can be used forregeneration of ZnS to ZnO. The advantageous results of the invention inreducing the rate of regeneration effluent recycled to the Claus plantcan be retained. Thus, when regeneration is conducted at a maximumtemperature in the regenerator of about 2100° F., then introducingabsorber effluent at a diluent rate of less than about 10% of theabsorber effluent rate is expected to reduce the size of the Claus plantcompared with the size where at least one Claus low-temperatureadsorption reactor is not used. Preferably, at these higher regenerationtemperatures, the rate is less than about 5%, most preferably less thanabout 3% of the absorber effluent rate of the absorber effluent leavingthe first absorption zone.

Economic comparisons of Configuration 1 and Configuration 2 plants asmodeled indicate that a Configuration 1 plant will cost about 1.2 timesmore than a Configuration 2 plant using less than about 10% absorbereffluent as diluent. Further savings can be achieved by using a threecatalytic reactor plant having Claus low temperature adsorption zones byfurther reducing capital and operating costs compared even to theConfiguration 2 plant.

The acid gas composition to the Claus plant itself will have littleeffect on the above results. The tailgas composition from aConfiguration 1 plant is a small function of its acid gas feedcomposition. For example, the above calculations are based on an acidgas feed having 69.0 mol. % H₂ S and no SO₂. The total feed to the theClaus plant, including the regeneration effluent stream, has an H₂ S+SO₂content of 27.3 mol. %. Without the regeneration effluent stream beingreturned to the Claus furnace, the tailgas from the Configuration 1plant contains 0.78 mol. % sulfur compounds as S₁, or 2.6% of the sulfurthat is in the Claus plant feed (a recovery of 97.4%). With theregeneration effluent stream being returned to the Claus furnace, thetailgas from the Configuration 1 plant contains 0.71 mol. % sulfurcompounds as S₁, or 3.8% of the sulfur that is in the total Claus plantfeed (a recovery of only 96.2%). Therefore, the large change inconcentration of H₂ S (and SO₂, if any) in the Claus plant feed makesonly a minor difference in the sulfur content of the tailgas, eventhough the recovery may decline significantly. The effect of acid gasfeed composition on the tailgas composition from a Configuration 2 plantis even less. As for the Configuration 1 plant, the above calculationsfor a Configuration 2 plant are based on an acid gas feed having 69.0mol. % H₂ S. The total feed to the Claus plant, including theregeneration effluent stream, then has an H₂ S+SO₂ content of 54.3 mol.%. Without the regeneration effluent stream, the tailgas from theConfiguration 2 plant contains 0.17 mol. % sulfur compounds as S₁, or0.6% of the sulfur that is in the Claus plant feed (a recovery of99.4%). With the regeneration effluent stream, the tailgas from theConfiguration 2 plant still contains 0.17 mol. % sulfur compounds as S₁,or 0.6 mol. % of the sulfur that is in the total Claus plant feed (stilla recovery of 99.4%). Therefore, reducing the fraction of the absorbereffluent that is used as regeneration feed diluent below about 35% (thevalue that creates the same regeneration effluent rate as a plantwithout low-temperature Claus reaction zones) can be expected to reducethe regeneration effluent recycle to less than that required in theabsence of a Claus low-temperature adsorption zone, due to a reductionof the amount of sulfur compounds in the Claus tail gas, independent ofthe acid gas composition.

The invention will be further described and further advantages andapplications and equivalents will be apparent to those skilled in theart from the description of FIGS. 1 and 2.

Referring now to the drawings and specifically to FIG. 1, FIG. 1represents an embodiment of the invented process in which absorption ofH₂ S by the metal oxide absorbent can be carried out at a temperatureabove about 1000° F., preferably in the range of about 1000° F. to about1200° F.

An acid gas stream 110 containing H₂ S is introduced into a Claus plantfurnace 112 and combusted, in the presence of oxygen containing gas, forexample, atmospheric air (source not shown), and/or SO₂ (provided, forexample, via line 111), to produce elemental sulfur, SO₂, and water. Theelemental sulfur is recovered and unconverted H₂ S and SO₂ are processedby Claus catalytic sulfur recovery 114, including at least one Clauscatalytic reaction zone operated above the sulfur dewpoint and at leastone low-temperature Claus adsorption reaction zone. Elemental sulfur isthus produced and removed, for example, by sulfur condensers (shownschematically by the arrow S). A Claus plant effluent stream is removedby line 116 containing sufficient reducing equivalents for reduction ofsulfur containing compounds remaining therein to H₂ S in thehydrogenation zone or in the absorber zone.

The Claus plant effluent stream in line 116 can then be heated to aneffective temperature as described herein. Preferably at least a portionof the heating requirements can be met by passing the Claus planteffluent stream 116 in direct heat exchange with the absorber effluentstream in line 156, for example, in recuperator 158, as indicatedschematically by the line marked A. Following heating in recuperator158, the heated Claus plant effluent stream can be provided by the linesmarked B to heater 117 for further heating to above 1000° F., preferablyin the range of about 1000°-1200° F. Alternatively, of course, the Clausplant effluent stream 116 can be provided directly (as indicated by thedashed line) and can be heated in heater 117 to a temperature in therange of about 1000° F. to about 1200° F. and introduced by lines 125,126, valve 126V, and line 130 into first absorber 134. That otherprovision can be made for heating the Claus plant effluent stream inaccordance with the invention will be clear to those skilled in thisart.

First absorber 134 contains a ZnO absorbent effective to absorb H₂ Spresent in the inlet stream to produce a sulfided absorbent and toproduce an absorber effluent stream 138 containing, for example, lessthan about 50 ppm H₂ S. Simultaneously with absorption in first absorber134, after heating to a temperature in the range of 1000° F. to 1200°F., SO₂ present in Claus effluent stream 116 can be hydrogenated to H₂ Sutilizing reducing equivalents present in Claus effluent stream 116 andthe resulting H₂ S can also be absorbed by the absorbent.

The absorber effluent stream 138 can be conducted by lines 142, valve142V, lines 152, 156, heat recuperator 158, and line 160 for discharge,for example, to the atmosphere. The heat recuperator 158 provides atleast a portion of the heat required for heating the Claus planteffluent stream as described above, or for producing high pressuresteam. A portion of the absorber effluent stream can be withdrawn fromline 152, by way of, for example, line 154, having valve 154V, fordilution of atmospheric air 172, via compressor 170 and line 168, havingvalve 168V, to produce a dilute air regeneration stream 166. Duringregeneration, valves 154V and 168V control the recyle rate to the Clausplant. In this way, recycle of regeneration effluent from theregenerator to the Claus plant can be reduced to a fraction of whatotherwise is returned.

The regeneration stream 166 can be heated in heater 174 to regenerationtemperatures and can be conducted by lines 176, 178, 180, valve 180V,and line 132 to second absorber 136 shown on regeneration. The heatedregeneration stream 176 is thus passed in contact with sulfidedabsorbent in second absorber 136 to produce a regeneration effluentstream 146 having a reduced O₂ content and an increased SO₂ and/orsulfur content. Stream 146 is conducted by line 144, valve 144V, heatrecuperator 190, compressor 192, and line 111 to the Claus plant furnace112. Alternatively, the regeneration effluent stream can be introducedinto a catalytic zone in the Claus plant 114 as indicated by dotted line111'; however, operation should insure that no free or molecular oxygenis introduced thereby into the catalytic zone.

Absorption is continued in first absorber 134 and regeneration iscontinued in second absorber 136 until prior to or just before H₂ Sbreakthrough occurs in effluent stream 138 from first absorber 134.Preferably, the oxygen content and regeneration stream flow rate isestablished so that the regeneration time (plus purge and slack time) isequal to absorption time prior to H₂ S breakthrough. H₂ S breakthroughcan be determined by monitoring the H₂ S content of first absorbereffluent stream 138 until H₂ S content can exceed a predetermined limitwhich can be, for example, that suitable to meet emission requirementsfor discharge of stream 160.

Following H₂ S breakthrough, first absorber 134 can be placed onregeneration and second absorber 136 can be placed on absorption byclosing valves 126V, 142V, 180V, and 144V in their respective lines 126,142, 180, and 144; and by opening valves 128V, 182V, 140V, and 148V inthe respective lines 128, 182, 140, and 148. Valve 194V in line 194(which can be closed during normal operation) can be utilized tominimize pressure shock during valve switching.

Prior to interchanging the first absorber and the second absorber, purgeof the second absorber zone can be effected by discontinuing O₂ flow tothe second absorber, for example, by closing valve 168V, and bycontinuing flow of absorber effluent by line 154 and valve 154V to thesecond absorption zone 136 for a period effective to reduce SO₂emissions, upon interchanging the absorbers, to a desired level.

Referring now to FIG. 2, FIG. 2 represents a second embodiment of theinvention in which absorption can preferably be conducted, for example,in the range of about 600° F. to about 1000° F. and having ahydrogenation zone prior to H₂ S absorption. The reference numerals forFIG. 2 are the same as for FIG. 1 except as may be indicated below.

Claus plant effluent stream 116 can be heated in reducing gas generatoror heater 118 and optionally reducing equivalents can be added toproduce stream 120. Stream 116 can also be at least partially heated inindirect heat exchange with absorber effluent stream 156 in recuperator158 as discussed above in reference to FIG. 1. Stream 120 can beprovided to hydrogenator 122, in which SO₂ (and other sulfur compoundssuch as elemental sulfur, COS and CS₂) present in the Claus effluentstream 116 can be hydrogenated to H₂ S over an effective hydrogenationcatalyst, preferably, for example, a cobalt-molybdenum hydrogenationcatalyst. The hydrogenated stream 124 can then be introduced, forexample, into first absorber 134 and H₂ S contained therein absorbed.

During regeneration of, for example, second absorber 136, regenerationand purging can be conducted as described above in reference to FIG. 1.

Other aspects of FIG. 2 and the operation thereof have been describedabove with reference to FIG. 1 and will not be repeated here.

It will be appreciated by those skilled in the sulfur recovery art thata Claus plant tailgas cleanup process is provided which is not sensitiveto water content in the effluent stream and which is capable of reducingthe volume of regeneration effluent returned to the Claus plant. Otherembodiments and applications in the spirit of the invention and withinthe scope of the appended claims will be apparent to those skilled inthe art from the description herein.

What is claimed is:
 1. In a process for the recovery of sulfurcomprising (a) introducing a H₂ S containing stream into a Claus plantproducing Claus plant effluent; (b) during an absorption periodintroducing Claus plant effluent into a first absorption zone containingZnO absorbent and reacting at least H₂ S with ZnO producing sulfidedabsorbent ZnS and absorber effluent; and (c) concurrently introducing O₂and a fraction of first absorption zone effluent into a secondabsorption zone regenerating ZnS to ZnO and returning effluent to theClaus plant, the improvement comprising:(1) lengthening the absorptionperiod in the first absorption zone by introducing a feed consisting ofClaus plant effluent having H₂ S:SO₂ in a 2:1 molar ratio into at leastone Claus catalytic reaction zone operated under conditions includingtemperature effective for depositing a preponderance of sulfur formed onClaus catalyst to thereby produce effluent provided to the firstabsorption zone having a reduced content of sulfur compounds relative toClaus plant effluent; (2) decreasing rate of return of effluent to theClaus plant due to (i) a reduced content of sulfur compounds in feedprovided to the first absorption zone resulting from step (1) combinedwith (ii) a reduced fraction of absorber effluent being provided to thesecond absorption zone regenerating ZnS to ZnO, said reduced fractionbeing less than 20% of the absorber effluent, and lengthening theregeneration period in the second absorption zone correspondingly to thelengthened absorption period in the first absorption zone andconcurrently introducing O₂ in minimum amounts necessary for completeregeneration of ZnS to ZnO in the lengthened regeneration period.
 2. TheProcess of claim 1 wherein step (b) is carried out at a temperature inthe range of about 1000° F. to about 1200° F. and step (c) is carriedout using a regeneration stream having an inlet temperature of about1000°-1200° F.
 3. The Process of claim 1 wherein the stream resultingfrom step (1) provided to the first absorption zone is maintained at aratio of hydrogen sulfide:sulfur dioxide therein of about 2:1.
 4. TheProcess of claim 1 further comprising at the end of the regenerationperiod and prior to H₂ S breakthrough in the first absorption zone,discontinuing introducing O₂ into the second absorption zone andintroducing an effective reducing gas stream into the second absorptionzone for a time sufficient to condition the catalyst and to eliminate orsubstantially eliminate a temporary increase in SO₂ emissions above abaseline level otherwise observed when the second absorption zone isreturned to absorption, then interchanging the flow of the Claus planteffluent stream from the first absorption zone to the second absorptionzone, and continuing the process.
 5. The Process of claim 1 wherein thefraction of absorber diluent introduced into the second absorption zoneregenerating ZnS to ZnO is less than about 10% of the absorber effluentexiting the first absorption zone.
 6. The Process of claim 1 wherein themaximum temperature in the second absorption zone is maintained duringregeneration at a temperature less than 2100° F.
 7. The Process of claim1 wherein the maximum temperature in the second absorption zone ismaintained during regeneration at a temperature in the range of1400°-1500° F.
 8. The Process of claim 1 wherein the maximum temperaturein the second absorption zone during regeneration is maintained at about1450° F.
 9. A process for the recovery of sulfur comprising:(a)introducing a H₂ S containing stream into a Claus plant comprising athermal reaction zone and at least one Claus catalytic reaction zoneoperated above the sulfur dewpoint; (b) producing and removing elementalsulfur in the Claus plant and producing an effluent stream, saideffluent stream having hydrogen sulfide and sulfur dioxideas gaseouscomponents in about a 2:1 molar ratio of H₂ S:SO₂ ; (c) introducing theeffluent stream into at least one Claus catalytic reaction zone operatedbelow the sulfur dewpoint and under conditions, including temperature,effective for depositing a preponderance of sulfur on Claus reactioncatalyst therein and producing a Claus plant effluent stream having areduced content of sulfur compounds; (d) introducing the Claus planteffluent stream resulting from step (c) into a first absorption zonecontaining ZnO absorbent, optionally after converting sulfur species inthe Claus plant effluent stream resulting from step (c) to H₂ S, andreacting at least H₂ S with the absorbent to thereby produce sulfidedabsorbent and absorber effluent; (e) concurrently introducing O₂ dilutedby a fraction of said absorber effluent, said fraction being less thanabout 20% of the absorber effluent, into a second absorption zonecontaining sulfided absorbent for a regeneration period in which ZnS isconverted to ZnO and producing regeneration effluent and returningregeneration effluent to the Claus plant, the moles of O₂ introducedduring the regeneration period being about equal to 3/2 times the molesof absorbed sulfur atoms plus a stoichiometric amount for combusting thehydrogen and carbon monoxide in said absorber effluent used as diluentto water and carbon dioxide wherein the rate of returning regenerationeffluent to the Claus plant is reduced in the presence of step (c) dueto (i) the reduced content of sulfur compounds in feed being provided tothe first absorption zone combined with (ii) a reduced fraction ofabsorber effluent being introduced into the second absorption zoneduring the regeneration period; and (f) discharging the remainingportion of said absorber effluent as an exhaust gas.
 10. The Process asin claim 9 wherein the fraction of absorber effluent introduced into thesecond absorption zone during the regeneration period is a fraction lessthan about 10% of the absorber effluent from the first absorption zone.11. The Process as in claim 9 wherein the fraction of absorber effluentintroduced into the second absorption zone during the regenerationperiod is a fraction less than about 3% of the absorber effluent fromthe first absorption zone.